A conventional positron CT apparatus (hereinafter called simply PET (Positron Emission Tomography) apparatus) will be described by way of example. The PET apparatus includes a detecting device having gamma-ray detectors arranged in a ring form for detecting gamma rays, and an image processor for producing an RI distribution image such as a sectional image or 3D image based on coincidence data obtained from results of detection by the detecting device. Each gamma-ray detector includes a scintillator having a two-dimensional arrangement of scintillator chips which emit light upon incidence of gamma rays, and a photomultiplier tube for converting the light emitted by the scintillator into electric signals. And a patient placed in the hollow of the detecting device is administered with a radioactive agent labeled with a positron-emitting radioisotope (RI: Radio Isotope). The positron-emitting radioisotope distributed in the body emits two gamma rays in 180-degree opposite directions. Each gamma-ray detector detects a gamma ray emitted out of the patient, and outputs an electric signal. The image processor produces an RI distribution image of the patient from data (hereinafter called coincidence data as appropriate) created when the electric signals are detected simultaneously (coincidence) in different positions (scintillator chips).
RI distribution images obtained from a whole-body PET apparatus having the above construction are useful for diagnosing mainly presence or absence, position, malignancy and so on of a tumor. However, since the hollow of the detecting device is about 65 cm to 90 cm in diameter, the RI distribution images have low resolution. Thus, it is usually difficult to determine tumors of 10 mm and less.
On the other hand, there are PET apparatus intended for diagnosis of only specific sites of interest of a patient, such as mammo-PET apparatus used exclusively for diagnosis of the breasts, for example. FIG. 11 (a) is a front view of a detecting device of a mammo-PET apparatus according to the prior art. FIG. 11 (b) is a view in vertical section thereof. FIG. 11 (c) is a view in horizontal section thereof. As shown, the detecting device 51 in a ring form has a reduced inside diameter (e.g. 18 cm), and gamma-ray detectors (not shown) arranged in the detecting device 51 are placed close to a breast (hereinafter referred to as “breast K1” as appropriate) which is a site of interest K1. Compared with the whole-body PET, this construction can discover and identify even a smaller tumor (see Patent Documents 1 and 2, for example).
[Patent Document 1]
Specification of U.S. Pat. No. 5,451,789
[Patent Document 2]
Specification of United States Patent Application Publication No. 2004/0183022